510 Prof. C. Y. Raman and Mr. V. S. Tamma on a 



be omitted, i. e. if k be zero, the amplitude graph will be- 

 'Come a strict parabola. There will be no upper limit to the 

 value of the amplitude, and therefore the collapse of main- 

 tenance will not be explained. If the frictional term in- 

 volving the first power of the velocity be omitted, i. e. if k' be 

 zero, there will be no explanation for the fact that there is a 

 lower limit to the value of a below which there is no main- 

 tenance. Facts (5) and (7) will be left unexplained. With 

 the two frictional terms all the facts are indicated. 



University College, Rangoon. 

 May 10, 1921. 



LYIII. On a New Optical Property of Biaxial Crystals. By 

 0. V. Kaman, M.A., Palit Professor of Physics in the 

 Calcutta University, and Y. S. Tamma, M.Sc, Lecturer in 

 Physics, Meerut College, India *. 



1. Introduction. 



.4 FACT of singular interest and importance in the optics 

 J~JL of crystalline media which appears hitherto to have 

 been overlooked is that a plate of a biaxial crystal bounded 

 by parallel faces is capable of focussing divergent rays pro- 

 ceeding from a distant light-source and forming real images 

 of it. Yery simple apparatus will suffice to observe this 

 phenomenon. The incandescent filament of a tiny 2-volt 

 lamp or an illuminated pin-hole serves as a suitable source of 

 light. At some distance from it is placed a crystal of 

 aragonite cut and polished with parallel faces at right angles 

 to the bisectrix of the acute angle between the optic axes. 

 On suitably orienting the crystal and examining the pencil 

 of light which has passed through it, a real erect unpolarized 

 image of the luminous filament may easily be picked up and 

 traced continuously away from the crystal for a considerable 

 distance. There is no difficulty in receiving and observing 

 the image directly on a plate of ground glass if desired. 



The image is sharp and bright and practically achromatic 

 if the object and the place of observation are both within a 

 few centimetres of the crystal, one on each side. As either 

 the object or the place of observation is drawn away from 

 the crystal the image spreads out into a spectrum. Using 

 monochromatic light, however, it can be seen that the image 

 remains quite well defined and sufficiently bright to be 



* Communicated by Prof. A. W. Porter, F.P.S. 



